most cells underego apoptosis if their dna is damaged. How can this be beneficial to an organism? someone pls help

It prevents the cell from passing on those defects to other cells through DNA replication in cell division. By self destructing before it gets the chance to divide, whatever problem the cell had ends right there and doesn't propagate in the organism.

the answer to your question is tooth decay.

after a the power stroke, adp is released, but the cross-bridge formed is still in place. atp then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site.